fix #70571 fix #93116 [GP6] crash when importing gpx ottava

[AntonioBL]

No description provided.

[AntonioBL]

This PR solves the crash and the import of ottava lines.
Tests added.

I tried to solve also the memory leak of "ottava" array variable. If this implementation is correct, maybe something similar could be done for "slurs" array (memory leak as well).

The ottavae in tablature look different than in GuitarPro, but this is another issue.

I had to open another PR because the rebase and forced updated were preventing an update of the PR. I don't know why, maybe because I first closed the PR and then force-updated the branch.

[AntonioBL]

It works also in 2.1, but a conflict merge must be solved in order to backport this PR (since types and subtypes changed with respect to 2.x branch).
In 2.1 I see problems in the second note of tied chords in the original example from issue #93116, but that is another bug.

[lasconic]

Thank you for looking into this. If you do have it working in 2.1, feel free to make a PR on this branch as well.